Electrolytic reversible color display device

ABSTRACT

A REVERSIBLE COLOR DISPLAY DEVICE UTILIZING A POROUS MEMBER IMPREGNATED WITH AN ELECTROLYTIC INDICATOR HAVING A PH RANGE VERY NEAR THE PH AT WHICH THE INDICATOR UNDERGOES A COLOR CHANGE. THE IMPREGNATED POROUS MEMBER IS PLACED BETWEEN A PAIR OF ELECTRODES ON OF WHICH MAY BE A WRITING ELECTRODE SUCH AS A STYLUS AND THE OTHER A BACKING ELECTRODE. UPON THE APPLICATION OF CURRENT TO THIS STRUCTURE COLOR MARKS ARE MADE ON THE POROUS MEMBER. THE COLOR CHANGE IS REVERSIBLE BY APPLYING A CURRENT OF THE OPPOSITE POLARITY TO THE DEVICE.

p 9 H. D. LEES ETAL 3,692,659

ELECTROLYTIC REVERSIBLE COLOR DISPLAY DEVICE FileqjAu 28, 1970 2Sheets-Sheet '1 VIEW FIG. I

A'H/////////////////////J FIG. 2a

INVENTORS HAROLD D. LEES PAUL F. EVANS MARTIN S. MALTS EDWARD W. VIPONDATTORNEY Sept. 19, 1972 H. D. LEES ET AL 3,

ELECTROLYTIC REVERSIBLE COLOR DISPLAY DEVICE Filed Aug. 28, 1970 2Sheets-Sheet 2 ADDRESS CKTS 5 FIG. 3

United States Patent 3,692,659 ELECTROLYTIC REVERSIBLE COLOR DISPLAYDEVICE Harold D. Lees, Henrietta, Paul F. Evans, Pittsford, Martin S.Maltz, Rochester, and Edward W. Vipond, gush, N.Y., assiguors to XeroxCorporation, Stamford,

oun.

Filed Aug. 28, 1970, Ser. No. 67,731 Int. Cl. C2311 5/68 US. Cl. 204-2241 Claim ABSTRACT OF THE DISCLOSURE A reversible color display deviceutilizing a porous member impregnated with an electrolytic indicatorhaving a pH range very near the pH at which the indicator undergoes acolor change. The impregnated porous member is placed between a pair ofelectrodes one of which may be a writing electrode such as a stylus andthe other a backing electrode. Upon the application of current to thisstructure color marks are made on the porous member. The color change isreversible by applying a current of the opposite polarity to the device.

This invention relates to a display device wherein a color change iseffected by a change in pH of an electrolytic indicator when a currentis passed through the device.

BACKGROUND OF THE INVENTION Along with the information explosion therehas arisen a need to create devices to channel data incoming from amyriad of sources, such as computers, into some form of manageableorder. Foremost among information handling devices are visual displaydevices which permit ahnost instantaneous viewing of the transmittedinformation. An electrolytic indicator display can satisfy the need forinformation handling devices.

An electrolytic indicator solution will undergo a color change when itsacidity is altered. Indeed, the pH of a solution connotes the acidity ofthe solution. A pH value below 7 which is neutral indicates an acidcondition while a pH above 7 indicates an alkaline state. The colorchanging property of indicators or electrolytics has been utilized inrecord mediums known in the art. However, in these prior art devices asolution is prepared that undergoes a chemical reaction resulting in achange in pH which in turn causes a color change characteristic of thecompound in the solution. For example, US. Pat. No. 3,402, 109 to Bermanet al. teaches that when a record medium or sheet is impregnated with anelectrolytically conducting solution containing a compound selected fromthe group consisting of absorbic acid and erythorbic acid with a silveranode placed on one side of the sheet and a cathode on the other side ofthe sheet, a dense blue-black mark is formed where the paper iscontacted by the anode. Silver ions are released into the paper duringcurrent flow between the anode and cathode by electrolytic erosion ofthe anode. These ions are reduced to metallic silver ions by the actionof the absorbic or erythorbic acid leaving a blue mark on the paper.This process produces a color change on the impregnated paper by theresults of two distinct processes; namely, a reduction process orchemical reaction and a pH change resulting from the reduction process.

We have discovered that we can produce a color change of an indicator ina solution having a pH range very nearly the value of pH at which itchanges color by passing a current through the solution. Thus, it is notnecessary to precede the change in pH with a chemical reaction. In ourinvention a change in current polarity also gives rise to a reversiblechange in color of the indicator.

Accordingly, it is an object of this invention to provide a displaydevice which creates a visual effect brought about by a change in the pHgradient of the indicator.

It is a further object of the invention to provide a record mediumimpregnated with an electrolyte or indicator which alters its colorstate solely as a result of current flow therethrough.

A further object of this invention is to provide an electrolyticindicator which has enhanced storage time.

Yet another object of this invention is to provide a soft electrolyticdisplay device which can reverse its color state and at the same timeelectrically control the color obtained.

BRIEF DESCRIPTION OF THE INVENTION There is provided in one embodiment adisplay system comprising an indicator cell in which a porous memberlayer such as filter paper impregnated with an indicator or electrolyteoverlays another porous member layer without an indicator orelectrolyte. This structure is then placed between two electrodes thatare connected to a voltage source by means of a reversible polarityswitch. The electrode overlaying the impregnated layer may be madetransparent so that a change of color thereon may be viewed. In anotherembodiment of the invention the transparent conductor may be replacedwith a writing electrode such as a stylus matrix or character wheel. Inyet a third embodiment the impregnated layer may be formed into a beltor drum and placed in a hermetically sealed liquid vapor filled housinghaving a viewing window. A stylus electrode is placed within the housingand forms characters on the belt as it rotates. In each of theseembodiments when a current is passed through the electrodes and porousmember, a mark is made on the impregnated porous layer due to a changein pH gradient resulting from the current flow.

In order to gain a better understanding of the invention, as well asother objects and further features thereof, reference is made to thefollowing detailed description of the invention to be read inconjunction with the accompanying drawings, wherein:

FIG. 1 shows a cross sectional view of an indicator ce 1;

FIG. 2a is a cross sectional view of an indicator cell showing acharacter wheel writing electrode;

FIG. 2b is a view similar to FIG. 2a with the addition of a secondporous member;-

FIG. 3 is a cross sectional view similar to FIG. 2a showing a stylusmatrix writing electrode; and

FIG. 4 is a side view showing a porous member in the form of a beltimpregnated with an indicator.

Referring now to FIG. 1, there is shown a schematic cross sectional viewof an electrolytic cell generally shown at 10 incorporating theprinciples of the invention. A layer 12 which may be an absorbent orporous sheet of filter paper impregnated with an indicator such as acompound of phenolphthalien overlays another absorbent layer of filterpaper 13 without an indicator. Layers 12 and 13 are disposed in anelectrically conductive relation between electrodes 11 and 14. Bothelectrodes 11 and 14 may comprise thin films of electrically conductivematerial such as tin foil or the like. In addition, electrode 11 may bemade of a transparent glass with a conductive coating such as is knownin the art. Where electrode 11 is made of a transparent material, thecell is viewed from that side. However, it is within the purview of theinvention, as it will also occur to those skilled in the art, thatelectrode 14 may be made transparent as well. Furthermore, layer 13 maybe omitted entirely. Electrodes 11 and 14 are connected by wireconductors 17 and 18 respectively to a reverse polarity switch S1. Thereverse polarity switch S1 connects a source of potential 15 acrosselectrodes 11 and 14.

Cell 10 is activated by wetting the porous layer 13 with a solutionhaving a pH very near the value of pH at which the indicator in layer 12changes color. Where phenolphthalein is the indicator, ordinary tapwater is used as an activator solution. When switch S1 is closed acurrent of approximately 60 ma. at a voltage of approximately 20 voltsis passed through cell 10 and an internal pH gradient is set up acrossthe cell causing layer 12 impregnated with the indicator to revert to ared characteristic pH color state. If the direction of the currentthrough cell 10 is now reversed, the pH gradient will change theindicator layer 12 to its neutral characteristic color. By utilizing pHindicators which go through a number of color changes at various pHranges it is possible to set and control the color of the cell byaltering cell current. Once the color has been set the current can beremoved and the cell will remain in this color.

The following table lists some examples of indicators. However, theinvention is not intended to be limited to the examples specified and itwill occur to those skilled in the art that other indicators could beused in practice of the invention.

TABLE I Approximate pH range Color change Indicator Bromophenol blue 3.-4. 6 Yellow, red or blue images on white Thyrnolphthalin.. 9. 3-10.5Blue-black image on white background.

Porous layer 12, if a red image is desired, may be impregnated withphenolphthalien by (l) preparing a 1% by weight solution of methylalcohol and phenolphthalien; (2) spraying layer 12 with the solution andallowing the alcohol to evaporate; and (3) allowing the sensitized layer12 to be activated by deionized or ordinary tap water.

If a blue image is desired a cell may be constructed in the mannerdescribed above but employing bromophenol blue as an indicator. Abromophenol blue indicator is yellow in the acid or low pH range ofapproximately 3.0 and is a deep blue in the neutral pH range ofapproximately 7.0. Bromophenol blue also has a narrow transition rangewherein a slight change in pH will cause the color to shift reversiblyfrom yellow to red to blue.

In preparing a display having a blue image a solution of dilute ammoniumhydroxide and bromophenol blue is mixed and hydrogen chloride is addedup to the point at which the solution begins to turn red. The solutionis then poured onto a porous member or filter paper and allowed to dry.The dry filter paper is then cut and placed into a cell. When current isapplied to the cell the indicator turns yellow. Reversing the polarityof the current causes the cell to turn blue. When the current throughthe cell is reduced, applying an opposite polarity pulse causes the cellto revert to its red neutral color.

Similarly, other sensitized sheets can be prepared in the mannerdescribed using cresol red and thymolphthalien.

In FIGS. 2a and 2b, there is shown an indicator cell wherein one of theelectrodes of FIG. 1 is replaced by a writing electrode. Referring toFIG. 2a, character wheel 20 which has alphanumeric symbols about itsperiphery contacts porous member or filter sheet 21 impregnated with anindicator solution of the type described. Porous member 21 in turnoverlays electrode layer 22. The electrodes 20 and 22 are connected tothe negative and positive terminals respectively of a DC. power source23. Writing speeds up to 85 inches per second have been demonstratedusing a writing electrode pulsed with a 30 volt pulsed oscillator.

The image retention or storage time of the display system is a functionof several variables but mainly dependent upon the stylus or electrodecurrent. Any variables which will affect the current supplied to thewriting electrode will, in general affect the image density and storagetime. For example, if the writing electrode pressure is increased or thewriting electrode voltage increased, the image retention or storage timewill be increased.

FIG. 2b is a view similar to FIG. 2a with the addition of a secondporous layer. In addition, FIG. 2b illustrates a means of obtainingeither a temporary or permanent copy. Specifically, there is shown inFIG. 2b a writing electrode or character wheel 20 and a first porousmember 21 which may be a layer of filter paper having such high porositythat bleeding or running of the marks on the sheet is eliminated. Filterpaper of this kind may be obtained from the Millipore Corporation,Bedford, Mass., under the name Millipore filter. A second porous member21a is disposed between the first porous member 21 and an aluminum backelectrode or platen 22. The second porous member may be soaked in a 10%solution of sodium chloride (NaCl). This process slows down image decayon the first porous layer 21 and provides a temporary copy thereof.However, if layer 21 is dried soon after imaging a permanent copy isproduced. An alternate method of forming a temporary image is to rollthe negatively biased character Wheel over the porous layer wet withordinary tap water. Another alternate method of forming a permanent copyis to wet the porous member with a salt solution as described. Informing either the temporary or permanent image in this alternatemethod, the latent image is developed after the wetting step by sprayingon a pH indicator. Phenolphthalien is used for a red image andthymolphthalien is used for a blue image. In the case of the permanentcopy, the paper is then quickly dried.

The addition of a metallic salt such as sodium chloride to the activatorsolutions of the impregnated member extends the image storage timeconsiderably. Without the salt the maximum storage time is approximatelyfive minutes when the writing electrode current has been adjusted toprovide maximum image density.

In FIG. 3 the character Wheel is replaced by stylus 30 which maycomprise a seven pin matrix. Porous medium 32 impregnated with anindicator of the type already described overlaps backing electrode orplaten 33 which may be composed of a conductive material such asaluminum. Address circuits 35 furnish pulses of the proper polarity todriver circuits 34. In driver circuits 34 the pulses are amplified andapplied to the proper pin electrode of the stylus 30 predetermined bythe signal from the address circuit. As a result of the current fiow,visible dots in the form of an image are made upon the porous member 32because of the change in pH gradient of the impregnated porous member.

Turning now to FIG. 4, there is shown yet another embodiment of theinvention. A belt 40 is comprised of a porous member impregnated with anindicator such as phenolphthalien or other indicator materials citedabove and is enclosed in a hermetically sealed water vapor filledhousing 44. The water vapor Within housing 44 provides a source ofmoisture for porous member 40, so that a solution may be formed with theindicator material within it. A window 43 permits viewing of the belt 40as it is rotated in a clockwise direction about rollers 41 and 42. Alsocontained within housing 44 is a stylus 46 which contacts belt 40 at theperiphery of roller 41. Roller 41 is grounded as shown to complete theelectrical circuit between it and stylus 46. Suitable conductors connectstylus 46 to a source of potential 57 through switch S4. This embodimentof the display system uses the natural image decay mechanism for theerasure of images made by stylus 46. The formed characters are broughtinto view as the belt rotates about rollers 41 and 42 and image decayoccurs on the opposite side of the belt out of view.

From the foregoing, a display device employing a color reversibleindicator system has been disclosed.

What is claimed is:

1. A display device comprising:

a belt of a porous medium impregnated with an indicator having a pHrange very near the value of pH at which the indicator undergoes a colorchange and capable of changing to a first characteristic color whencurrent is permitted to flow through it in one direction and to anothercharacteristic color when current is permitted to flow through it in theopposite direction,

a first roller and means for applying ground to said first roller,

a second roller spaced from said first roller,

said belt enwrapped around said first and second rollers,

a writing electrode touching the periphery of said first roller andtouching the outer side of said belt and forming an electrical circuitwith said belt and said first roller,

a housing containing a water vapor sealed therein and enclosing saidwriting electrode and said belt en- References Cited UNITED STATESPATENTS 1,970,539 8/1934 Bausch 204-2 1,068,774 7/1913 Hutchison 204-224699,784 5/1902 Brooks 204-2 OTHER REFERENCES Handbook of Chemistry andPhysics, 32nd ed., 1950, pp. 1482-1485, Chemical Rubber Publishing Co.

JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl.X.R. 204-2,

